Correlation between clinical and sonographic assessment of the fetal head station in the second stage of labor: a prospective observational study

Background: A correct estimation of the head level is required to evaluate labor progression and decide how to best expedite birth – i.e. instrumental vs cesarean delivery – if dystocia or fetal distress are diagnosed during the second stage. However, evidence from the literature has shown that the clinical diagnosis of the fetal station is mostly imprecise. Intrapartum transperineal sonography has been proposed as a tool allowing a reliable and reproducible evaluation of the fetal head station. Objective: The aim of this study was to investigate the correlation and agreement of the clinical and sonographic assessment of the head station in the second stage of labor. Methods: Single-centre prospective observational study involving a non-consecutive series of singleton pregnancies enrolled during the second stage of labor. Eligible cases were submitted to vaginal examination performed by a senior Obstetrician for the assessment of the station of the fetal head. Following such clinical assessment, transperineal sonography was performed to evaluate the sonographic station by measuring the angle of progression (AoP), which was considered the reference method for the assessment of the head station. The relationship between the clinical station and the sonographic station was evaluated by Pearson's correlation, and the agreement between clinical and sonographic station was investigated by comparing the clinical findings and the AoP width across clinical station according to previously published reference ranges. Results: 303 cases were included. Moderate-to-fair correlation between the clinical and sonographic station was demonstrated in all cases (0.647, p < 0.01). An agreement on station level was recorded in 94/303 cases (31.0 %) with the highest percentages noted for the sonographic stations −2 (5/11, 45.5 %) and −1 (16/34, 47.1 %). Head engagement was clinically diagnosed in 208/303 cases, of whom 195 had sonographic confirmation of head station at or below 0 (93.8 %). Conversely, among the 95 cases clinically diagnosed with unengaged fetal head, only 36 (37.9 %) were sonographically confirmed as non-engaged. Overall, the sensitivity, specificity, PPV and NPV and LR + and LR- of the clinical assessment for head engagement were 76.8 %, 95 %CI(71.1–81.8), 73.5 %, 95 %CI(58.9–85.1), 93.8 %, 95 %CI(89.6–96.6), 38.9 %, 95 %CI(28.1–48.4), 2.89, 95 %CI(1.81–4.63) and 0.32, 95 %CI(0.24–0.42), respectively. Conclusions: Findings from our study show a moderate-to-fair correlation between clinical and sonographic head station. An agreement between clinical and sonographic diagnosis of head station occurs in less than one-in-three cases overall and particularly in the event of unengaged head. Indeed, a clinical diagnosis of head engagement corresponds to a truly engaged fetal head in over nine out of ten cases, whilst a clinical diagnosis of unengaged fetal head is sonographically confirmed in one out of three cases.